1. Field of the Invention
The invention relates to a wireless IP phone, and more particularly to a wireless IP phone supporting both a wireless local area network (WLAN) standard and the General Packet Radio Service (GPRS) standard.
2. Description of the Related Art
The Wi-Fi standard comprises a series of standards for wireless local area network, including the IEEE 802.11a and 802.11b standards. Because of the prevalence of the wireless local area network, the Wi-Fi standard is extensively adopted by industry.
Many standards for transmitting audio data over the Internet have been established. The control signals of ordinary IP phones are exchanged through H.323, SIP, or MGCP standards, and the voice data of ordinary IP phones is exchanged through the user datagram protocol (UDP) of the TCP/IP protocol. The control signal packets and voice data packets of a wireless IP phone are exchanged with an access point of a WLAN through one physical layer standard of the Wi-Fi standards. However, the user of a wireless IP phone often moves during a telephone conversation. The physical layer connection between the access point of the WLAN and the wireless IP phone tends to break off if the user moves beyond a distance of 300 feet from the access point if the connection is made through the IEEE 802.11b standard.
Because the mobile communication is prevalent all over the world, the Global System for Mobile communication (GSM) networks have been established. The GSM network, however, is a circuit switched network, and cannot be used to directly transfer Internet data packets. Thus, a General Packet Radio Service (GPRS) technique is provided to transfer Internet packets through GSM networks.
A data packet of a IP phone can be transmitted through different physical layer standards. Thus, the voice data packets and the control signal packets of a wireless IP phone can be transmitted through a WLAN complying with Wi-Fi standards or the GPRS network. Because the physical layer connection between the access point of the WLAN and the wireless IP phone tends to break off when a user moves, the invention substitutes the WLAN with the GPRS network when the signal strength of the Wi-Fi physical layer connection is weak, and the voice data packets and the control signal packets of the wireless IP phone can still be transmitted through the GPRS network.